Chromite ore pigment



Patented Feb. 18, 1947 GHROMITE ORE 'PIG'MENT Gordon Derby Patterson, Wilmington, 'DeL, and Clifford Kanne Sloan, Thornton,Pa., assignors to E. I. du Pont de Nemours & Company, Wil- .mington, D el.,,a corporationpf Delaware a N0 Drawing. Application April 28,1943, Serial, No. 48;,878

9 Claims. 1

This inventionsrelates .to pigment-useful sub- ;stances, and has :more particular reference to novel-metal protective pi ments adapted for use in .various coating compositions.

Metal protective pigments have long been used i hepriming coat of or anic film-forming com- ;positions'applied to thesurfaces of metallic 0b- .jects. They have been-effective, at least in part, becauseof a specific inhibiting or passivating;action exertedat the surface of the metal. ,.Among the pigments .usefulin this 'field are red lead and certain chromium-containing materials. ,How- ,ever, these products have :.not met .all .practical requirements, judging :from the wide use that is .made of :iron oxide, va relatively non-inhibitive but cheap type of pigment. Among .deterrents'to the use of red leadare its paint instability, chalking, and high cost as used. The ,use of prior chromium-containing pigments also has been characterized :by high costs due largely .to the many expensive and time-consuming steps necessary in their manufacture. For instance, these chromium-containing metal protective pigments are usually prepared by filtering, drying and further processing a precipitate obtained from a wateresoluble alkali chrcmate or bichromate obtained in turn after a number of purification steps, following the leaching of a mass obtained by roasting soda ash, chromite ore and lime. Furthermore, practical use of the prior art chromium-containing pigments has 'demonstrated that their .protective action in paint films on exposure is not, satisfactorily effective. "It is apparent that the inhibitive action of the chromate constituents of these pigments is not sufliciently available .for as long atime as is to be expected of a protective coating.

Prior art-materialshave been obtained with a view to securing a usable chromate material by heating chromite ore. Thus, ordinary com- ;mercial processes for production of chromate depend on roasting chromite ore with 'both lime and a more active alkali, such as soda ash. 'If the latter is omitted, the yield of hexavalent chromium chromiumcompound thatis formed is not sufficiently soluble to make the processa practical one for commercial chromate production. Accordingly, attempts to prepare chromates by heating chromite ore and lime have been handiis considerably reduced and the i :2 making the resultant material impractical for metal protective pigment purposes.

We have discovered that certain high temperature reaction products of alkaline earth-oxides and chromite ore prepared in accordance with our invention are directly useful as imetal'procappedby the necessity of added processing, such tective pigments, and, in fact, have unexpectedly high value in imparting anti-corrosion properties to films of organic coating compositions applied to metal surfaces. This high protective performance i surprising in view of the relatively low hexavalent chromium content'of our prodnot, as compared with commercial chroma'tecontaining pigments such as chrome yellow, zinc yellow and the like. 1

Accordingly, a principal object of this invention is to overcome the disadvantages of such prior chromium-containing pigments and to p'rovide novel pigmentsubstances generally useful -'-in coating formulations and particularly efiective for use as metal protective pigments in organic film-forming vehicle adapted to be applied to metallic surfaces, especially those subject "te -corrosion. A further object is to prepare a chromium-containing pigment, the hexavalent chromiumv component of which has a maximum opportunityfor exerting an inhibitive action during'exposure on a metal surface. A still further object is to prepare our pigments by a simplified process whereby one can obtain a highly useful and efiective product at relatively low cost. 0ther'objects and advantageswill be apparent from the ensuing description.

These objects are attainable'in thisdnvention by our novel pigment compositions, adapted --'for control of corrosion, andw hich -conta-in,' as-jthe essential active pigment ingredient, 'a reaction product obtained by subjecting an intimate mixture of a chromite-ore and'an alkaline'earth'material-which is heat-decomposable to the oxide, to treatment at a relatively high temperatureln the presence of anoxidizing medium, followed by disintegration "of the dry reaction product which results to a state of fineness suitable for pigment use.

More specifically, the invention comprises an improved metal protective type of pigment prepared by subjecting a finely-divided, intimate mixture of chromite ore and an alkaline-earth material, such aslime, to roasting treatment-at a relatively high temperature in the presence of oxy enor an oxygen-containing gas, followed by disintegration of the resulting dry reaction-mass to desired pigment particle size fineness, such care and control being exercised over the ratios,

0029 being usually suflicient. ;emciency of: the mill, we prefer to employ the ,for removal of undesirably large particles.

' integrated by treatment, first in a jaw crusher,

and then in a ball or other grinding or pulverizing mill, to reduce the ore to a stateof relatively fine particle size. This finely-divided or powdered material is then intimately mixed with controlled amounts of an alkaline earth compound, such as calcium oxide or hydroxide, also in relatively finely comminuted state. Intimate association of the pulverized ore and alkaline earth material is readily effected by resort to conventional mixing apparatus, such as by means of a short ball milling treatment, and while the reactants are in wet or dry condition. Resort to this mixing and grinding treatment serves the desired purpose of controlling both the subsequent reaction of the mixture and the size of the pigment particles jultimatelyrecoverable therefrom. After desired homogeneous association of the reactants, the mixture is roasted, under controlled conditions,

and in the presence of an oxidizing agent, in a rotary or other conventional type of furnace. jPreferably,;roasting temperatures ranging from about 1050 C. to about 1150 C. are utilized with the oxidizing agent comprising gaseous oxygen or air. The extent of the roasting is largely governed ;by the time required for supplying the oxygen 1 component of the reaction, the temperatures used, th extent of hexavalent chromium conversion desired, and by the pH and .otheressentialproperties which the final pigment is to exhibit. Usually, roasting periods of fromabout thirty minutes to an hour suffice for the reaction on a small scale andin-an electrically heated furnace. In large scale operations and where an internally-fired furnace .is used, from about 2 to about fihours ;at the indicated temperatures may be required, due principally to the diificulty of maintaining a pigment, can be employed. Thus, it can be used as a component of greases, non-drying oils, fatty acids and similar compositions for the protection of metallic surfaces against corrosion. Due to its low cost, it can, if desired,be employed without a'vehicle and as a dry packing material for corrodible. objects for transit.

To a clearer understanding of the invention,

the following specific examples are given, each being in illustration but not in limitation of the invention sufiiciently high concentration of oxygen in the V combustion gases at the higher temperature used. After the desired roasting treatment, the'alkaline earth-chromium ore reaction product is allowed to cool, and is then directly converted into a pi ment by disintegration through treatment in conventional disintegrating apparatus. A hammer mill of the Mikropulverizer? type can be effectively used for this purpose, one or twopasses through the mill with a screen opening of 0.020-

To promote the same in circuit with an air classification system V The product WhlCh is recovered, due to its low cost,

' its color and hiding power characteristics, will ;be generally useful as a pigment and as the sole pigment substance of a film-forming composi- 1 tion. It is particularly useful as the primer pigment in films of organic coating compositions for the protection of metallic surfaces, especially ;those subject tocorrosion. Thus, it is usefully 3 effective intheprotection of iron and. steel surfaces and of non-ferrous metals, including aluminum, zinc, magnesium and the like, as well as alloys, particularly light-weight alloys, such as those of aluminum and magnesium, especially the Ltypes used for aircraft construction. The reaction product can also be used in anyway inwhich a P ment, and more particularly an inhibiting Emmple I Twenty-eight hundred and eighty pounds of water are run into a wood vat equipped with a paddle agitator. Six hundred and forty-eight pounds of lump lime are slowly added with constant agitation so as to avoid overheating during the slaking process. The slaked lime is then fed as a water slurry by gravity to a- 1200 gallon pebble' mill, the'charge being augmented by 792 pounds of pulverizedTransvaa1 chromite ore having a CrzOa content of 42%,"the ore being previously pulverized to pass through a 100- mesh sieve and 85% through a 325-mesh sieve. The charge in the ball mill is then subjected to a 3-hour grinding period at which time 97.2% of the suspended matter passes through a 325- mesh sieve. The ground slurry is then pumped to filter frames where the Water content is reduced by half to give a press cake having 50% solids. The filter cake is approximately 1%; inch in thickness. The press cake is dried overnight in an oven heated by 20 pounds steam pressure.

Sixteen hundred and seventeen pounds of material is handled at this stage, the change in weight being largely due to the water taken up during slaking of the lime. The press cake lumps are then charged into a batch rotary furnace preheated by internal firing with fuel oil. The furnace is 10 feet in length, 8 feet in internal diameter, and rotates once each minute. The furnace is equipped with thermocouples at both ends, permitting accurate controlof temperature during theroast. After a 2-hour period required for bringing .to temperature, the furnace is held at temperature for 3 hours, the temperature reading being 1150 C. at the front or combustion end of the furnace and '1100 C. at the exit end of the furnace. Fuel oil is supplied to the burner at 40 lbs/sq. in. pressure with a consumption of 17 gallons per hour. In addition to'the primary air used in the combustion of the oil, an auxiliaryair line is used to supply extra air at the rate of cubic feet per minute in order to accelerate oxidation. The roasted product is then discharged through a pair of manholes in the furnace into a buggy equipped with water-cooled coils. The product at this point is in the form of small, friabl'e,'marble-like lumps averaging inch in diameter. For conversion to final pigment form, the dry reaction product is disintegrated by passing through a Mikropulverizer equipped with a 3 screen having openings of 0.020 inch. The recovery is 1394 pounds, the overall losses in processingbeing approximately balanced by the oxygen taken up during oxidation of the chromite. The resulting product is useful as a metal protective pigment.

The resultant pigment has a hexavalent chromium contentof 14.4% (calculated as CrOs), an acid-insoluble residue of 9.5%, a pH value of 11.50

(unboiled slurry) and of 11.65 (boiled slurry),

and aresidue of 5% on a,325 -mesh screen when sprayed thereon with mineral thinner.

Example. II

A pigment of general usefulness as a component of metal protective coating compositionfilms under ordinary atmospheric conditions; is pre-- pared in thefollowing manner.

One hundred twenty-five parts ofchromite ore having; a CrzOa content of 41.5% and dryground to pass a IOU-mesh screen is. added toa pebble mill along with 125" parts of lime and 500 parts of water and the mass ground for-16 hours. The chromite/lime slurry is then filtered and the resulting press. cake dried overnight in an ovenat 100 C. The dry intimate mixture is then roasted in'an electric furnace at 1000 C. with air passing through the furnace. After roasting 3.5 hours, the reaction product is removed from the furnace and cooled. The dry reaction product is- 97.5% solublein 1:1 nitric acid. It'contains 19.2% hexavalent chromium calculated as CIOa and has a pH of 11.2.

ExampleIII ternally heated in an. electric furnace with air supplied. to the contents at a rate of 1 cu. ft./min. Each batch is brought to .a roasting temperature of 960 C. and held at temperature 3 hours and discharged in lump form. The batches'are combined and mikro-pulverized" using a screen hav.

ing- 0.029 inch openings. The finished pigment product has a hexavalent chromium content of 18.9% calculated as CrOa, is- 97.5% soluble inni trio acid, and has a pH of 11.2- when slurried in water. It has a density of- 3.42. V

The resultant pigment prepared entirely by dry reactions is useful as a metal protective. pigment; Thus, after incorporation at 40% byvolume in a linseed oil film-forming coating composition and after application to a vigorously wirebrushed prerusted' iron surface, the system is in excellent condition after a years outdoor1ex-posure to the elements, whereas. a similar. surface coated. withan ironoxide control paintis corroded after-identical exposure. it A corresponding system with red lead as the sole pigment shows moderately severe corrosion under identical exposure' conditions.

Example IV A roughly pulverized mixture of 151 parts of:

- demonstrated from its excellent behavior on outdoor exposures, particularly exposures in warm, humid climates. Thus, the average grading of a Example I? Two hundred seventyeiour parts. of barium oxide and 137 parts. of. chromite. ore (42% CrzO; content). are ground, two. hours in a. pebble mill with water. The ground; slurry is filtered and dried and roasted 2 hours atll00 C. with ready access of air. The resultant product is pulverized to pigment form in which it is useful in organicfi'lm -forming coating compositions. The pigment has. a hexavalent chromium content of 16.2% (calculated as CIOs), has a pH of 11.9 when tested' in water, and is 82% soluble in nitric acid.

The general usefulness of the pigment ofzthis invention as. a metal protective pigment is clearly number of exposures of steel surfaces protected by an'alkyd' fortified linseed, oil vehicle composition pigmented 40% by volume with, for instance, thev dry reaction product of Example I, is considerably superior to that of a corresponding iron oxide-pigmented system after ayear and a half .outdoor' exposure. This is illustrated by the following :table of resultsfrom such an exposure:

chromite ore and 243 parts of limestone ('equivalent to 136 parts freelime) is ball milledfove r night with 200 parts of water. The ground slurry.

is filtered and. dried. The dry lumps arethen roasted at 1000 0. for an hour with ready access to air. The roasted mass is mikro-pulverized through a 0.029 inch screen to convert-to pigment form.- The product has ahexavalent-chromium content of 17.8% calculated as-cros, is 97% solublein nitric acid, and hasa pH of 1114 when tested as. a water slurry. Theinexpensi-ve: product serves as a highly'efficient pigment whenin corporateda, in: organic: film-iorming com-positions ueedin primingmetal.surfaces. v

- Iron oxide:

Gralding of re a we w freedom.

. from rusting Chrome ore-lime reaction product 8.14 Iron oxide 4. 0'

The significance ofthe code used in reporting, extent of corrosion is as follows, .a difference of 1.0 point in the grading being materialz Condition Grading I No rusting.

Slight rusting;

Considerable rusting, Severe rusting. 1

Intermediate conditions are graded proportion; ately'. i I i An al yd-fortified linseed oil aintpr pare in a similar manner except that a part of thef chrome ore-lime reaction product pigment is're-' placed by iron oxide and talc to give a 10 /15 ratio by weight of reaction product/iron oxide/talc. is superior in performance to paints prepared by a corresponding-extension of prior art metal protective pigments. v grading of a number of panelscoated with this system is superior after exposure to thatof panels coated with similar systems in which the. chrome ore reaction type productof Example I is replaced by an equal, volume ofiron oxide or zinc yellow as the; prime pigment. The average, gradingsare' 7 as follows:

, 'Gradingof 1 relative freei dom from -Prime,pigmcnt corrosion ChromeQOrc/lime reaction product Zinc yellow 1.

1 seed. oil. coating: compositions. to! previously rust Thus, the average ings of I Q rosion, are as follows:

metal protective effectiveness sitions, said hexavalent chromium content ranges I from about 8% to about 21%, with optimum rei ed and wire brushed iron surfaces and exposure i of the painted surfaces for one year in a subtropical climate; the pigment of Example I shows 1 marked advantages in'protection of the underlying metal against corrosion; The relative gradthe-three systems, as todegree of cor t (hiding of Prime pigment ti g h g ff' V r t t corrosion A; liromeore/lime reaction product 7. 5 B. Zinc yellow 4. 8 1 C. Iron oxide i 2.0

" As statedin practicing the invention such care is exercised as will insure a final product suitable ijor an intended or pigment use. Particularly, care. is takenthat the product will exhibit certain essential, values in, respectto hexavalent I chromium content, pH,'acid-insoluble residue, color ,jandjparticle size. i

U I For examplefthe alkaline earth/chromite ore dryreactionproduct orpigment'v of this invention should contain at least 1.5% and up to, say, 30%, of hexavalent chromium, calculated as CrOa (determined by dissolving the pigment in nitric acid 1 and titrating with a standard ferrous ammonium sulfate solution) sults-accruing when thecontent is about 12% andbetween and 18%.. These preferred pig- 1 ments are generally obtainable by (1) using a relatively low alkaline earth/chromite ore ratio (e. g., a lime/ore ratio of,'say, 45/55 instead of 50/50), (2) a longer raw grinding period, andv (3) higher roasting temperatures (1050 0. 1150 0.).

a, definitely lower pH value than thatof the free I I alkaline earth oxide, corresponding to the alkaline, earth material from whichthe products are.

be obtained by. controlling a number of variables, including: (1) using a higher alkaline earth material/chromite ore ratio, (2) shortening the pe-I riod of grinding of the unroasted material, and (3). roasting at a relatively moderate temperature 950,C.-1050? C.) By, such methodsand by use of selected chromite ore gradesandlof higherox-I ygen contentduring roasting, CIO: values up to the limit of the indicated, preferred range can be obtained. Products with higherCrOs content are I not preferred, however, because they exhibit somewhat lessened durability characteristics on exposure tohigh moisture conditions, in compari-; son with pigments prepared of somewhat lower hexavalent chromium content and, pH value.

Preferably, and for marked" in coating compo- Such pigments are more resistant to cracking on; the paint film to unusually high; moisture conditions and characteristically haveeta-.

aftepS O-GO minutes intermittent agitation, at,

room temperature, of a slurry containing 1 gram of thereaction product pigment in 100 cc. of water. Further assurance that the pH'is not too :high can be had by making a second test on such a slurry by boiling for one hour,and determining the pH of the slurry-after again cooling to room temperature. Under these conditions of test, free line gives a-pH reading of about 12.3.

Theproduction. of a pigment low in pH value can be brought about (1) V by using smaller amountsof the alkaline material in theroast mixture, 2) by grinding the raw pigment components a longer time, and .(3) by roasting longer and at a higher temperature. If, however, raw

' pigment grinding is excessive, the resultant pigment will be undesirably low in hexavalent chro-.

with a higher ratio (such as 50/50).

mium 'content and unnecessarily high in acid-insoluble residue. For a lower alkaline earth/ore I ratio (such as /55 lime/ore); this overgrindingdifficulty is morelapt to be encounteredthan With the /50composition, the longer grinding period is preferred because of its efiect in lowering thepH oi .the final product.

' The acid solubility of the reaction productmaterial is also'useful in selecting conditions for preparing satisfactory pigments. Acid-insoluble residue is expressed in terms of the percentage of the pigment remainingv insoluble after digesting. a .few minutes in warm 1:1 nitric acid. The com:

mercial chromite ore is made more acid-soluble as the course of the roasting proceeds. With'a 50:50'lime/ore'ratio the .final' product is almost completely solublepi. e.,' its acid-insoluble residue valuewill be of the order:'of,0--2%.

, residue of 0% may be due to an exact balance beprepared. Higher hexavalent chromium; contents (beyond the indicated preferred range) can 7 about 5%. to 10%.

. 'As'indicated,it is desirable that the pI-I'of the reaction product be definitely lower than that of the free alkaline earth used in its manufacture.

Preferably, and in order to insure film durability,

the pH value is at least from 0.3 to 0.5 units belowthat given by the free alkaline earth oxide,- if presentas such. in thefinished product. Generally'thepH may range irom.about'8;9 to below about.12.3 (say, to :about12.0) ;'a preferred range being from10.8 to 11.8, with 11.4 being optimum.

A product containing appreciable amounts of undesirably basic calcium compounds which, when tween the lime andchromite ore ingredients,'we'

prefer, in obtaining a product useful for general pigment purposes, to so adjust the conditions of 'manufacture that the final product will give an acid-insoluble residue value of at least 1%, and not to exceed about ,A. most usefully operative range comprises .fromabout-'2% to about 15%,while an optimum range comprises/from Maintaining a residue value: at the'lower limitof at least'1%, can be conven iently had :either by slight reduction in the.

amount of lime added, or by increasing the periodofgrindin of the unroastedmixture, Using a lower lime/ore ratio such as 45/55 affords a higher acid-insoluble residue value in the final product, e. g., of the order of 8% to 15%. Products; having -a 90% residue result from use of a highchromite ore concentration. These, how- -ever,.jexhibit decreased general utility as metal vprotective pigments. Since generalmetal protective utilitydecreases as the acid-insoluble residue increases, we prefer. to utilize in. the final:

product the indicated 2% to 15% acid-insoluble residue values. The water solubility of our preferred pigments will be found to be relatively low. For example 7 if a test sample is slurrie'd with water at 20 C. for

ofthe material is dissolved.

. Although diminution of the acid-insoluble contentis useful in determining theflcourse of the- 5 minutes with vigorous agitation, less than 0.2%

Although a soluble fraction reaches a minimum desiredjlevel.

The color of the final product isalso important and is of considerable use in following the course of the reaction as well as determining the usefulness of the .product as a P gment. The lime/chromite ore reaction product pigmentsmay vary in color from fidd fih brown ,-to .blacl; to

to kht green col r usual is tive of too i h n in t a alkaline .ea t imr o, t o ar ast, o-r-too lowajrq st .e t mpe u n nced eddishu a y d c tive .of an exces qfunr a tedchrqmite ore. A reddish brown co o can al o result if insufficient oxidizing conditions prevail during roasting. Preferably, .We-so adjust the amount of lime that we obtainour Preferred, relatively black colored pigments, indicating thatneither ore nor lime is in excess. These preferred pigmentsishow a characteristic selective absorptionof light .over the .Wave length :range 4000-7000 A. as .is shown by examination with va.-1'.ec ording spectrophotometer. Thus, said Preferred productszusually conform :to the following limits:

(1) 'The ratio f is not more than2..,20

%zreflectance., at 4000A 2 The ratio Wienermorethanuo reflectance at.4000

a) The ratio w is between .0. 8 0 and 1,20

% reflectance at 6000 A 1 Particle size of the final product'is also of considerable 1 importance in determining --its usefulness for general pigment purposes andespecially as a 'pigmenting ingredient'in metal protective paints. In general, the final produ ct should be t a ly e 9 a tic es lar e than -2 i o sa le 90% .orits part sle fshould pas 3 mesn and the grit va ue .o iths ewiii ment retained by m s re n shou d beles than .Preferahlmthe major..po'rfltiori of .the pigment is below '5 micr.ons',.with',a substantial fraction thereof beingasilowas 0.5 to Bitiicrons, e. g., at least 95% .of the particles shouldpa ss .325 mesh, andits grit value should be vless than .5 An optimum type of J pigment affords va 99% passage through325meshandthas agrit value of less and after roasting, a vpigment.h avir g'theseatisfactory particle size ,and ot'her values will be obthan 2%. By .resortto the,grindingstepsbemre tained. As alreadyv stated,.one pass;.of1theroasted material through a Milrropulverizew isusually suficientto-reduce thematerial. to astate ,of-subdivision suitable for pigment .use. .{Ifhe residue retained in the 325-mesh screen test is somewhat governed by the screen used in the. ilVIikropulverizer disintegration. With a .0 020 ,inch. screen, the 325-mesh residuemay be of the .orderof 1 to 5%, Whereas with a 0.029 inch screenit is about 2 to 10%. With an air. c1assification.u t inseries.

with the grinding unit, even are 'terdiminutionpr partic e. size. can b o tained..ieggstiess than .112%

er the roduct ein tain by a 2 uiie h sc ssnfitness fi ure iar .sem'ewhat de endentoi p ed by th us o cer ai .r aa c mm li pre s, theironpxidelcontentof-Which is as high as A tho h o s v ne a H ent n i llii t ative of different typesfofiutilizable chrom se ss hih c a ac ri t anal se a ve in the indu tr a M ns a a ocks A erican ins itute of Min ng n Met lu al sinecrslQBi. pa e 1205,. ar .th i ll wing:

Analyses per cent Source-0f chromiteore 0 ,0, FeO A1203 s1o,- CaO MgO Brazil 42.6 13.8- 14.6 7.4 2.1 45.9 Canada... 31' 1 6. 8v ;10.2' 8.13 71.0, 17.9 Cuba... .3 3 14.3 26.0 5-6 0. 8 11.2

v =31s5 13.9: 26.6 as 1.2- 17.7 no.5 14.2215 6.l 0.9 18.3 Greece 1115.6, 21.51 4. 0. 1 16.8 "se.1 15.4 22.4 5.2 0.2 16.0 383 15. 21.6 1.7 0.6. ,;16.5 La1apan 2i (Africa) 46.6 19.0 13.6 4.7 0.4. 1 1.7 -Philippines 32.1 13.0: 27.6 5.3 1.-1 18.2 Y 1 54.;1 4. 1s ;2. 5 0.35; 1 6.1 Bhei esielei iee) 4.5.4 .1 1. 1 .3 7- M. 1. l 4.41 15 12:9 7. 7 41-5 14.3 .46.;5 2 .4 -7.op.9 a.7 Russia. 39.1 14. .6 7.17 1.2 16.4 m-o 14. .3 10.1 o.4 16.a .46.2 15.; ,s- 4.0 1.3 5.4 Serbia 41.712 5' 12 0.9 15.9 mransvaalJ(Africa). L -1' 24 "5 -'3.-6 '08- 11.6 44.9 25 I I 2.;2 as, 10.5 15.0. 25.7 15.1 11 11:7 19.8 l-urkey 47.01 12.5 13.5 7.2 1.3 19.2 1 .4115 12.; 1 1. ,o. 1-1.9 0.9 0.0 ;";i6.'6 1259 12 5 e 7. 1. 2 17.3

Qerta nsbu cssof dom sticpre a s canbeus d, particul rly ai er qncentrat n .q the lor iiiim component- 1 Qocimi ution pf the com o ents, of th ch 9- imite ore and a k linerearth materials p ior to roasting is .an essential lpart f'thisimlention. .ltis necessary that the tn rticle .size or the rawpigmentzingliedients .be reduced at .this point :to insure satisfactory ,uItimateLpigment particle size, as well .as isufficient chemical reaction uring the roasting process. The intimate ;mixtu ije .of the chromite ;ore .and alkaline .earth. material vnan be .accomplished either :during or subsequent to the finalgrinding .of the componentparts. .As stated,

.such intimate mixing andgrindingsteps haverthe dual :purpose of controlling .both reaction and -suitable .size .of pigment particles. mixture can :beaccomplished during a, short ball .The intimate millingcperation with the lime slurry and pulverizedrchromite ore. 'When the components, are wet ground, it canbe accomplished simplypby thorough wet blending. The wet blend can-be filtered and dried prior .to roasting or the.:wet

cake can ..be fed directly to :the roaster. If the If the intimatemixture of the ingredients .is-obstained by dry grinding, it is'bften advisable" that the resultan material. .be com acted i t roa n sa d qm act ng .be n'e ac plis ed .0-%.r ange ar a t u y tedfo se, 1 h he 0.1 .l w re a r s an be m lo edleither in the laststages of the v itself or in an added pressure compacting or dry step briquetting operation. Inlarge scale operations,

's'ufficient compacting may occur in the roasting process during rolling of the charge in thefurnace.

" 'Most, if not all, of the comminution of ore necessary for obtaining bothreaction during roasting and the suitab-lyfine particle size for pigment V 7 use can be conveniently carried out prior to mixing with the 'alkaline'earth material. The chroa mite ore is most economically separately processed from the lump formto a finely-divided powderjmost of which passes through a 325-mesh screen. In efiectingthis, a series of dry grinds through jaw crusher, disc attrition, and steel ball mills usually sufficeto reduce the ore to a stage where allof the material passes through a100- meshscreenand about 85% passes'througha 325- mesh screen.

To obtain material, is continued until all but about 1-5% of the material passes a 325-jmesh screen. I v As thealkaline earth component of the reaction with the chromite ore may be used any compound or mixture thereof which is in oxide form 1 or is thermally decomposable to the alkaline earth" oxide at or below the temperature being usedin the roastingloperation. Specifically useful oxides include those of calcium, strontium and bariu'mg Examples of other useful compounds are the various, alkaline earth hydroxides and salts, including, carbonates, acetates, and the like,

which decompose in the presence of chromite ore at or below the roasting temperatures used. Calcium compounds, such as calcium oxide, calcium hydroxide, andcalcium carbonate, are preferred because more adaptable for commercial applica- -tion.- Thus,flump'f-lime, air-slaked lime, waterslaked lime; and limestone serve as excellent starting materials. In the case of limestone,-the

"rock is desirably ground to a powderbeforethe final coniminutionwhich is carried out in'the same manner as that for the chromite ore above referred to. "In the case of lump lime, much of the necessary initial comminution isautomatically accomplished in a water-slaking operation which can be employed prior to final comminution by grinding.

' The ratio between the amounts of alkaline earth and chromite used depends upon a number of factors, including: (1) raw materials used, (2) amount .of grindingand (3) roasting time and temperature. The amount of alkaline earth ma-.

terialto be used isdependent on its basic equivalent. Although the ratios are conveniently expressed in terms of the ratio lime/chromite ore,

the ratios for other alkaline earth materials can readily be. calculated therefrom. Generally, the

ratio of lime may range from about, to about 70%, andis preferably from about 40% to, 50%. able to moisture conditions encountered in out For preparation of pigments that are durdoor exposures, lime/chromite ore ratios as high as 55 /45 can be employed by exercising'care dur-. ing processing. With the higherlime contents, such is ratios of 70/30, for example, a greenish rather than a durable type of pigment results.

On the other hand, when ratios as low as, say,

: 1.5/85, are used, the products are less useful from a metal protective viewpoint, due to a; lower Micronizing is a further practical means of subdivision. The final comminution is preferab1y,-but not necessarily done in the mix withthe alkaline earth material.

optimum effects, this final comminution', whether done in the presence or absence of alkaline earth hexavalent chromi 12 r 'u'm content and higher content of unattackedchromite ore. As noted, the ratiorange of fromf/60 toj50/50is most generally useful. It is desirable, however, that with the higher lime'rati'os the processingvariables be so adjusted that avoidance of free'lime in thefinished' product will be had. For example, when 'uslngas high a ratio as 50/50, it is usually necessary to grind the raw'materials to a greaterextent; as illustrated in Example II, and/or'conduct the roast at a higher temperature if a highly durable pigment is wanted. Therange in ratiobetween 42/58an'd 48/52 i s'preferred (a lime'content'of about 45% being optimum), especially When working with the ordinary grade of chromite ore averaging 42% to 48% in CrzOs content.

Although one of'the reactions involved in the roasting-step isthatof oxidation ofchromium *from the trivalentto the hexavalent condition, this is apparently but one of a number that occur. The best pigment need not necessarily exhibit the highest hexavalent chromium content. Most effective pigments canr-be prepared at temperatures above that at which maximum hexavalent chromium conversion isobtained. Thus, while temperatures in the-range of -1050 0.4150 C.

are preferred, andabout 1150-C. is optimum,

higher hexavalent chromium content can often be obtained at lowertemperatures In certain instances, such as when low lime ratios are used,

temperatures as low as 900-950 C. can be employed. Even at temperatures as low as 700 C.,

areactionoccurs, i. e., conversion to hexavalent chromium takes place, but since the reaction is very slow they are obviously of less practical utility. Higher temperatures, of the order of 7 .pf from 900 C. to 1200 C. Preferably, temperatures ranging fromvabout 10009 0. to about 1200 Glare used,'wi th optimum results accruing here- 035011200? 0. v y Y The time of 'throast is largely governed by in with the employment of temperatures of 1100".

that required for supplying the oxygen componentof the reaction. Whereas from one-half to one'hour usually'sufiices for the reaction in small scale operations in an electrically heated furnace, as much as 2 to 6 hours at the indicated i preferred temperature is'often required when operating the processinf aninternally-fired furnace on a relatively largegcommercial scalebecause of the difficulty [of maintaininga sufficiently high concentration of oxygen in the combustion gases at. the higher temperature used. In any caseythe course ofthe reaction can be followed by'testing progress samples for hexavalent chromium, acid-insolubleresidue, and pH,

and continuing the roastinguntil these values level off. Inasmuch as hexavalent chromium conversion is usually morecomplete at temperatures below that desirably used in the roast, it is oftenadvisable to supply oxygen to the charge on the down-heat, i. e.,,the charge is preferably not dumped until a temperature of 900 C. hasbeen reached, thesupply of air being continuedafter the fuel rate is decreased.

, 'When cooled, the alkaline earth/chrome ore reaction product isgdirectly converted into pigment form by disintegration. This can be carseries with an air classification device.

ried out by any well-known, conventional disintegration or grinding equipment, such as i'ball, pebble, rotary hammer, Raymond or ring :roll mills, '8zc. Use of a hammer'mill of the Mi'kropulverizer' type is very advantageous for this purpose and one or two passes through such a mill with a screen opening of DIED-10.029 inch usually suffices, especially if the grinding unit 'is'in For certain uses, it is advisable to resort to more vigorous disintegration methods, including micronization. This direct conversion of the roasted reaction product to pigment form solely by'a dry process is very desirable. Processing theroasted material by a, wet method, such as by wet pebble milling, while utilizable, is not preferred because it impairs the effectiveness of the resulting pigment'to some extent and gives aproduct of somewhat more limited application. Thus, an aftertreatment of the pigment with water prior to ex- ;posure in "the film gives a product that "is some derstood that the pigment has general usefulness in organic film-forming velncles and 'par ticularly in such systemswhen'usedin the protection of metal surfaces, Thus, the dry reaction product pigment is useful with various oleores- 'inous vehicles including -linseed oil,"China Wood oil, oiticica oil, and the like, nitrocelluloseand other cellulose derivatives use d"in.coatingicompositions, chlorinated rubber, alkyd and"alkydfortified oleoresinous systems, phenol-formaldehyde resins such as Bakelite and the like, Vin- .ylite, vinyl acetate and polyhydric alcohol-mixed esters of drying oil acids andothermonofunce 1 4 'overia "film pigmented with the reaction product of this invention.

Asjstated, the low cost, 0010!, and hiding power characteristics of our chrome ore reaction prodment can be suitably extended or augmented lithopone, etc.

be added'to the pigment to modify Water-sensiv 4O Rhodesian .ore, and a:-Philippine .ore.

with otherm'aterials such as are customarily *used "with prior "art primer pigments. Examples-of extenders used in this manner include -iron -oxi-de, ferrite yellow, "talc, a'sbestine, calcium sul- 5 fate, barium sulfate, various silicates, and the "like. Furthermore, the pigment can be "augmented by the addition of other prior art colored or white pigments, such as red leadfzinc yellow, calcium chromate, zinc sulfide, titanium oxide, If desired," also, organic agents such as the polar type, including materials of anionic and cationic characteristics, as contemplated in-the co-pending application SerialNo. 484,883, filed April 28, 1943, of c. K. S1oan,,ca n

tivity or dispersion control properties, such .as can stability, consistency, degree of hard caking, case of application,and the like.

The composition of the final alkaline Learith/ chromite ore dry reaction pigment is to some extent dependent uponboth the alkaline earth material used and the particular grade of .chromite ore employed. The following "table gives an analysis, calculated in term of oxidev content, of representative pigments prepared in accordance with the invention .from ,several particular types of chromite ore namely, a low grade Cuban ore, a Transvaal vore tofhigh iron contenlg The ,elementar-y analysis is given for tWO pi ments for each of thechromite ores, representing the compositionohtained on the onehand using-aa40/60 the same materials in a /50.ratio.

*Analysis"(%) *oipigment Ratio Type of Ore CaO/ore. Chromium calculated. .Fe Oa A1203. ,SiOz. CaO 'tMgO ,as 01203 V .Guban 40 60. .19. s, 9. 1e. 2 a. 6 411 10. 8 I. I 50 50 -l6.5 v.5 13:5 3 51 v 9 Philippine (high grade) 50/50 27.2; 7.0 .5.8-' 11.3: :50.3 .81) lransvaal (sample #1) 40/60 27 15.5 .9. 6 l. 2 41 l 6 /50 22.5 13' 3 1 '51 5 lransvaal (sample #2) "45/55 '26. 3; 1,4 7.5 1.19 "44.1 16.1 Rhodesian- 40/60. 28 8 .10 4. 5 .41 .8. 8 50/50' 23.5 6.5 '8. 5 3.7 51 "7. 5

:tlonal monocarboxylic acids. such as beta-furylr acrylic acid, deltaQA-hexadienoic ,acid, metha- :crylic acid, alpha-.vinylcinnamic. acid and the like, and synthetic resin vehicles generally.

Asis the .case in the use of other primer pigments, top-coats can beapplied over theprimer film to increase durability of the system containing the chromiteore reaction product pigment.

Uncoated primer film containing the product .of this invention is durable, however; thus the film .containing it.is usually found to be intact and serviceable at .a ,periodin exposure .When a red lead film is'badly chalked and-failinglby erosion. ."Conventional .blackrand aluminum topcoats are among the systems satisfactory for application cated in the above table.

It is to be emphasized that the preferred pigments of this invention are substantiallyfree of active alkali metal compounds, such as sodium, potassium,.and the like, although negligible traces of these compounds, such as might ordinarily be present in commercial chromite ores, are permissible.

.Because of the .wide diversity of;materia'ls en- ,Lteringinto preparation of "our novel dry reaction product pigments and the likelihood that .these characteristic lines are listed below. 1 will be evident that thecharacteristic ore lines protective action which the efi'ectiv components represent several rather than a single compound, it is diflicult to detera mine what materials in the chromite ore/alkaline .1 .earth reaction product pi ment are actually re- 1 sponsible for its unexpected excellent behavior, especially in metal protective pigment systems. It does appear, however, that the plurality of components are an effective combination for affording the superior performance attained.

Certain lines in the X-ray powder pattern which our product exhibits appear to be common to the preferred pigment.

and the characteristic calcium oxide and calcium carbonate lines are absent. We consider ,these characteristic lines responsible for the high are preferred pigment ex- Microscopic examination of our preferred pigments of the line/chromite ore reaction prod- 'uct indicates that the material largely comprises two types of crystals, one of which appears orange and the other bluish-green. These products are in the refractive index range of 1.73 to 1.83. Both materials are isotropic and appear to be somewhat variable in composition, as indicated both by slight changes in color and in refractive index from particle to particle which may be indie ative of the presence of solid solutions. These variations are especially striking in the case of the blue-green material which appears to be the major component of the preferred range of pigments. cium carbonate crystals /2% or less) maybe present. guished because of the birefringent character (refractive indexes of 1.49 and 1.66).

the slow reaction of air with the pigment at the lower temperatures after roasting is completed:

A number of. It

This trace of material can be distin- 50 This trace 3 of calcium carbonate appears to originate from In cases smal mount of cal- 1 certain I a s of from about 10.8 to about 11.8', a hexavalent' by gradual disappearance in .contact with Water. f

I It is likely that this material comprises the one 1 water.

'As noted above, the products of our invention are adapted for general pigment, use and are especially useful as the pigmenting substance in metal protective pigment or coating composition formulations comprising an organic film-forming binder. Said product will exhibit superior properties over prior art products, especially zinc yellow and basic lead chromate, Furthermore, our products have the advantage that they are 7 free from added alkali and also free from alkali 'chro nates, the presence of which is undesirable The 15 because of their water-sensitivity which may cause blistering of the protective coating.

We claim as our invention: 1. As a new black pigment composition, the calcined reaction product of a finely-divided mixture consisting of chromite ore and an'alkaline earth compound heat decomposable to the oxide, in the ratio between,50/ and.60/40, said product containing less than .2% of'water-sob; uble material being substantially free of active alkali metal compounds, in pigment particle size state of subdivision with the major portion of its particles being below 5 microns and at least 95% of its particles'being capable of passing a 325 mesh screen and having a pH value ranging from 8.9 to 12.0, a hexavalent chromium content ranging from 8% 'to 30%, calculated as C1O3, and an acid-insoluble residue ranging from I about 2% to 15%.

2. As a new black pigment composition, the dry, calcined reaction product of a finely divided mixture consisting of chromite ore, oxygen and calcium oxide, said ore and oxide being in the ratio between 50/50 and /40, said product containing less than 2% of water-soluble material being substantially free of active alkali metal compounds, in a pigment particle size state of comminution with the major portion of its particles being below 5 microns and at least 95% of its particles being capable of passing a 325 mesh. screen and having a pH value of from about 10.8 to about 11.8, a heXavalent chromium content equivalent to from about 10% to 18%, calculated as CrOs, and an acid-insoluble'residue ranging from about 5% to 10%:

3, As a new black pigment composition, the dry, calcined reaction product of a finely divided mixturecons'isting of chromite ore, oxygen and calcium hydroxide, said ore and hydroxide being. in the ratio between 50/50 and 60/40, said product containing less than 2% of water-soluble material being substantially free of active alkali metal compounds, in a pigment particle size state of comminution with themajor portion of its particlesbeingbelow 5 microns and at least 95% of its particles being capable of passing a 325 mesh screen and having a pH value chromium content equivalent to from about 10% .to 18%, calculated as CrOs, and an acid-insoluble residue ranging from about 5% to 10%.

4. As a new black pigment composition, the

dry, calcined reaction product of a finely divided mixture consisting of chromite ore, oxygen and calcium carbonate, said ore and, carbonate being in the ratio between 50/50 and 60/40, said product containing less than .2% of water-sob.

uble material being substantially free of active metal compounds, in a pigment particle size state of comminution With the major portion of its particles being below 5 microns and at least 95% 'of its particles being capable of passing a 325 mesh screen and having a pH value of from about ranging from about'5% to 10%.

10.8 to about 11.8, a hexavalent chromium content equivalent to from about 10% to 18%, calculated as CrOs, and an acid-insoluble residue 5. A process for preparing an improved metal protective pigment composition which comprises subjecting a finely-divided mixture consisting of chromite ore and an alkaline earth material heat decomposable to the oxide, in the ratio between 50/50 and 60/40, to reaction at temperatures ranging'from about 1000 C.-.1200 C. in the 17 disintegrating the resulting dry reaction product to a pigment fineness state of subdivision whereby the major portion of the pigment has a particle size below microns and at least 95% of its particles are capable of passing a 325 mesh screen.

6. A process for preparing an improved metal protective pigment composition which comprises subjecting a finely-divided mixture consisting of chromite ore and calcium oxide, in the ratio between /50 and /40, to reaction at temperatures ranging from about 1000 C.-1200 C. in the presence of a gaseous oxidizing agent, and then disintegrating the resulting dry reaction product to a pigment fineness state of sub-division whereby the major portion of the pigment has a particle size below 5 microns and at least 95% of its particles are capable of passing a 325 mesh screen.

7. A process for preparing an improved pigment adapted for use as the essential pigment ingredient in metal protective pigment compositions containing an organic film-forming binder, which comprises subjecting a previously-disintegrated, finely-divided mixture consisting of chromite ore and calcium oxide, in the ratio between 52/48 and 58/42 and all of which material is capable of passing through a. IOO-mesh screen with about thereof capable of passing through a 325-mesh screen, to calcination from about 30 minutes to about one hour, at temperatures ranging from about 1050 C.-1150 C., in the presence of oxygen, and then disintegrating the resulting dry reaction mass to a state where the same is freed of particles above 20 microns and at least 9.0% of its particles pass a 325-mesh screen.

8. As a new black pigment composition, the dry, calcined reaction product of a finely-divided mixture consisting of chromite ore, oxygen and calcium oxide, said ore and oxide being in the ratio between 52/48 and 58/42, said product containing less than .2% of water-soluble material being substantially free of active metal compounds, in a pigment particle size state of subdivision with the major portion of its particles being below 5 microns and at least of its particles being capable of'passing a 325-mesh screen, and having a pH value between 8.9 and 12, a hexavalent chromium content, calculated as CrOa, between 10% and 18%, and an acid insoluble residue ranging from about 5% to 10%.

9. A metal protective coating composition containing an organic vehicle and as the metal protective ingredient thereof the pigment composition of claim 1.

GORDON DERBY PATTERSON. CLIFFORD KANNE SLOAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Doerner State C'ollege of Washington, Mining Experiment Station, Bulletin V, Sept.

1939, page 6. V 

